Recently, heat-shrinkable plastic films made from a variety of resins are widely used for applications such as external packaging for the appearance improvement of packaged goods, packaging for avoiding direct collision of contents, and label packaging for both protection of glass bottles or plastic bottles and indication of commercial products. Among these heat-shrinkable plastic films, stretched films made from polyvinyl chloride resin, polystyrene resin, polyester resin, or the like are used for the purpose of labeling, cap sealing or assembling packaging in various containers such as polyethylene terephthalate (PET) containers, polyethylene containers, and glass containers.
However, although excellent in shrinkage characteristics, polyvinyl chloride films pose problems of, in addition to low heat resistance, generating a hydrogen chloride gas during incineration, generating dioxins, and the like. In addition, use of polyvinyl chloride resin films as shrink labels for PET containers and the like also causes a problem in that a label has to be separated from a container upon recycling the container. On the other hand, while good in finished appearance after shrinkage, polystyrene films are poor in solvent resistance, and thus suffering a disadvantage in that an ink of a special composition has to be used upon printing. Additionally, polystyrene films need to be incinerated at high temperature and cause a problem of generating a large amount of black smoke along with foul smells during incineration.
Therefore, polyester films exhibiting high heat resistance, easy to incinerate and excellent in solvent resistance have been widely utilized as a shrink label. The amount of use of polyester films tends to increase as the distribution amount of PET containers increases.
Moreover, usual heat-shrinkable polyester films that are greatly shrunk in the width direction are widely utilized. In such a case, heat-shrinkable polyester films the width direction of which is the main shrinkage direction undergo drawing at a high ratio to the width direction in order to exhibit shrinkage characteristics in the width direction. However, only drawing at a low ratio is often applied to the longitudinal direction orthogonal to the main shrinkage direction and there are also unstretched heat-shrinkable polyester films. Films to which drawing at a low ratio is only applied to the longitudinal direction or films which is stretched only in the width direction have a defect of being poor in longitudinal mechanical strength.
In addition, a label for a bottle should be attached to the bottle in a circular shape and then circumferentially heat-shrunk. Hence, when a heat-shrinkable film that heat-shrinks to the width direction is attached as a label, a circular body should be formed such that the width direction of the film may become the circumferential direction and then the circular body should be attached onto the bottle by cutting the circular body into pieces each having a given length. Therefore, it is difficult to attach a label made from a heat-shrinkable film that heat-shrinks in the width direction onto a bottle at a high speed. Thus, recently, films that can be wound directly around the circumference of a bottle and attached from a film roll (so-called, wraparound) and heat-shrink in the longitudinal direction are demanded. Additionally, recently, a wrapping method is developed that involves covering the surrounding of a one-side opened synthetic resin container such as a lunch box with a band-like film to thereby keep the container enclosed. Films that shrink in the longitudinal direction are suitable also for such packaging applications. Accordingly, demands for films that shrink in the longitudinal direction are expected to increase rapidly in the future.
In order to eliminate the disadvantage of mechanical strength in a direction orthogonal to the main shrinkage direction as described above and in order to exhibit a function of shrinking in the longitudinal direction, a heat-shrinkable polyester film is known that is made by stretching an unstretched film 2.0 to 5.0 times respectively in the longitudinal direction (also called a lengthwise direction) and the width direction (also called a transverse direction), and then re-stretching 1.1 times or more in the longitudinal direction to thereby exhibit shrinkability as well as control both the Young's modulus in the longitudinal direction and the Young's modulus in the width direction to be a given value or more (Patent Document 1).
Patent Document 1: Japanese Unexamined Patent Publication No. H08-244114